1. Field of the Invention
The present invention relates to a magnetic disc driving apparatus such as a driving device for a removable flexible disc, particularly, relates to a driving magnets and an allocation of coils in a spindle motor.
2. Description of the Related Art
Recently, a floppy disc drive (FDD) is required of a smaller and thinner profile, and further required of high performance. Accordingly, a spindle motor utilized for an FDD is required of a smaller and thinner profile and also high performance.
FIG. 3 is a perspective view of a 3-phase flat spindle motor for an FDD of 1/2 inch thick. The motor comprises the stator 103 composed of the metal-based printed circuit board 112 and the rotor 101, which is attached to the stator 103 with rotating freely.
FIG. 2 is a sectional view of the main part of the motor. Further, FIG. 1 is a plan view of the spindle motor shown in FIG. 3 with removing the rotor 101 and a bearing (not shown). In the case of a 3-phase driving motor, the relation between a quantity of magnetic poles and a quantity of coils is shown in Table 1 if quantities of the magnetic poles and the coils are 4 n and 3 n respectively, where "n" is an integer of more than one. A magnetization chart of the driving magnet and the allocation chart of coils are shown in FIGS. 4(a) through 6(b), in case that the "n" is from 2 to 4. FIGS. 4(a), 5(a) and 6(a) respectively show the magnetization of the driving magnet. FIGS. 4(b), 5(b) and 6(b) respectively show the allocation of the coils. FIG. 1 shows the example of 16 poles and 12 coils.
TABLE 1 ______________________________________ n 1 2 3 4 . . . ______________________________________ Quantity of magnetic poles (4n) 4 8 12 16 . . . Quantity of coils (3n) 3 6 9 12 . . . ______________________________________
The stator 103 is composed of the metal-based printed circuit board 112. There provided a plurality of flat coils 105 and 107 on the metal-based printed circuit board 112 in a ring. Each coil is annulately allocated by the electrical angle of (4/3).pi. and is adjacent to each other. These flat coils 105 and 107 are formed a fan shape of a narrower inner circumference and a wider outer circumference. Electric current flows through the coils and the coils function as driving coils for the rotor 101. The flat coils 105 and 107 are different from each other in a plain size. Each flat coil 107 of a smaller plain size is allocated between every 3 flat coils 105 of a larger plain size.
According to FIG. 1, 12 flat coils in total composing of 9 flat coils 105 of the larger plain size and 3 flat coils 107 of the smaller plain size are allocated with covering 360 degrees. A frequency generator (FG) pattern 104 is provided in adjacent to the outer circumference of the flat coils 105 and 107. 3 magnetic sensors or Hall elements 106 for detecting position are respectively provided in between the flat coil 107 of the smaller plain size and the FG pattern 104.
The plain size of the flat coil 107 is smaller than that of the flat coil 105 by design in order to secure the space for arranging the Hall element 106, which is utilized for detecting a magnetic pole position of the rotor 101, on the metal-based printed circuit board 112 inside the FG pattern 104. The flat coil 107 of the smaller plain size is allocated at equal intervals of 120 degrees to other 2 flat coils 107 respectively. A magnetic recording and reproducing head (magnetic head) 108 is allocated at the area facing toward the flat coil 107 of the smaller plain size.
A driving ring magnet 102 is provided on the rotor 101 with facing toward the flat coils 105 and 107 on the stator 103. An FG magnet 109 is provided on the rotor 101 with facing toward the FG pattern 104. An FG signal for controlling rotation is generated by the FG pattern 104 provided on the stator 103 and the FG magnet 109 provided at the outermost circumference of the rotor 101.
The driving ring magnet 102 is magnetized in 16 poles radially. A spindle 114 is fixed at the center of the rotor 101 and the spindle 114 is secured in the bearing (not shown) provided on the stator 103 so as to rotate freely.
According to the configuration mentioned above, since the Hall elements 106 are allocated in an adjacent area of the FG pattern 104 in order to detect a magnetic pole position of the rotor 101, a space must be provided inside the FG pattern 104 so as to lead out wiring from the Hall element 106 to the outside of the FG pattern 104. The FG pattern 104 is lacked to almost 2/3 of the total circumference in order to provide connecting patterns for the flat coils 105 and the Hall elements 106. As a result of lacking the FG pattern 104, an output of the FG signal decreases and an accurate signal can not be obtained. Accordingly, it causes some problems such that rotation accuracy of the rotor 101 is deteriorated.
Further, since it is necessary to manufacture the flat coil 107 of the smaller plain size in addition to the flat coil 105 of the larger plain size or a regular size, it causes another problem of increasing a cost of the stator 103 in order to manufacture at least 2 types of flat coils and to assemble them.
Furthermore, since the flat coil 107 of the smaller size is utilized, flux from a magnetic circuit can not be utilized sufficiently. It causes further subjects to be solved such that a torque and a torque coefficient, hereinafter called a Kt, are reduced. Moreover, since the magnetic head 108, which records signals on or reproduces signals from a magnetic disc, approaches the upper side of the flat coil 107 of the smaller plain size closely, the magnetic head 108 happens to detect noise from the flat coil 107 by way of the rotor 101 and it may cause a data error.
In addition thereto, in case that the driving ring magnet 102 of 16 poles or 8 pair poles is utilized, each magnetic pole pitch 110 is 360.degree..div.16=22.5.degree.. In case that 12 flat coils are allocated, a coil allocation pitch 111 becomes 360.degree..div.12=30.degree.. It is hard to secure sufficient coil width because neighboring flat coils come in contact with each other. On the other hand, in case that neighboring flat coils generate magnetic field of different poles respectively, a part of fluxes directly cancels each other and generated torque is deteriorated, since utilization efficiency of fluxes is decreased. By arranging the neighboring flat coils so as to generate magnetic field of the same polarity, fluxes generated by a driving coil are effectively interacted with a driving magnetic pole and torque performance is improved. Particularly, it is necessary for the neighboring flat coils to generate magnetic field of the same polarity in order to obtain necessary torque performance even though a thickness of a rotor and a stator is thinned. However, on the other hand, fluxes generated by the driving coil leak out from the rotor. The leaked fluxes may affect the magnetic head and interfere with reading and writing data. Further, the larger outer diameter of a motor, the more torque is generated, and the better motor efficiency is obtained because a total amount of fluxes of driving magnetic poles and winding of a driving coil are increased. Thus, it is effective to extend the outer diameter of the rotor as far as a vicinity of a moving area of the magnetic head. However, in this case, fluxes leaked out from the rotor as mentioned above seriously affect the magnetic head. Particularly, in case that a device is arranged in a thin profile, reading and writing data are interfered by leaked fluxes because a distance of a thrust direction between the magnetic head and the driving coil is shortened. Accordingly, it is necessary to enlarge the distance of the thrust direction between the magnetic head and the driving coil in order to solve the problem. Enlarging the distance is also another problem for thinning a thickness of a device